The present invention relates to a semiconductor substrate including wafer-like plate-shaped crystal and a compound semiconductor crystal layer formed on the plate-shaped crystal, a method of manufacturing the semiconductor substrate, a semiconductor device, such as a semiconductor laser diode used as a light source for a pickup for an optical disk and the like, a light emitting diode used as a light source for a display device and the like, and a field effect transistor, and a method of manufacturing the semiconductor device.
Recently, nitride compound semiconductors such as GaN, InN and AlN are in the limelight as a material for a short wavelength light source and an environment resistant device because such semiconductors are of direct transition type and have a large energy gap. For example, GaN has an energy gap as large as approximately 3.4 eV at room temperature, and hence is a promising material for a light emitting element for emitting light in a range between the blue region and the ultraviolet region.
In forming a film of nitride compound semiconductor crystal, metal organic vapor deposition (hereinafter referred to as the MOCVD) is generally adopted. In the formation of a film of, for example, GaN crystal, trimethylgallium and ammonia are used as the materials, and Ga obtained by decomposing trimethylgallium and N obtained by decomposing ammonia are adhered onto a substrate having been heated at a high temperature. Thus, a monocrystal film of GaN can be obtained.
At present, a sapphire substrate is generally used as a substrate for filming the nitride compound semiconductor crystal.
In a sapphire substrate, however, the lattice constants in the a-axis direction and the c-axis direction are 4.76 .ANG. and 12.99 .ANG., respectively, while those of the GaN crystal are 3.19 .ANG. and 5.19 .ANG., respectively. In this manner, there is large lattice mismatch between the sapphire substrate and GaN crystal, and therefore, threading dislocations in number larger than 1.times.10.sup.10 cm.sup.-2 are caused during the film formation by the MOCVD from the interface between the sapphire substrate and the GaN crystal toward the inside of the GaN crystal.
Furthermore, since the sapphire substrate and the GaN crystal have different thermal expansion coefficients, the threading dislocations can be grown or cracks derived from the threading dislocations can be caused within the GaN crystal during temperature increase/decrease between room temperature and a high temperature exceeding 1000.degree. C. in the MOCVD.
Since the threading dislocation can work as a non-radiative recombination center or can capture a carrier, the performance improvement of a light emitting diode can be obstructed by the threading dislocation. Also, when a light emitting diode is manufactured by using GaN crystal including a large number of threading dislocations, a leakage current can flow, or emission failure or device destruction can be caused due to degradation in quantum efficiency. In particular, when the threading dislocations are caused in a light emitting portion of the semiconductor device, the device destruction can acceleratingly proceed, resulting in largely decreasing the life time of the device.
As means for decreasing the threading dislocations, a method in which a buffer layer is inserted between the sapphire substrate and the GaN crystal is widely adopted at present. In this method, since a stress caused by the lattice mismatch between the sapphire substrate and the GaN crystal can be relaxed by the buffer layer, the occurrence of the threading dislocations within the GaN crystal can be suppressed. In addition, since a stress caused due to the difference in the thermal expansion coefficient during the temperature increase/decrease can be also relaxed by the buffer layer, the growth of the threading dislocations and the occurrence of cracks within the GaN crystal can be suppressed.
Furthermore, Japanese Laid-Open Patent Publication No. 4-297023 describes that a buffer layer of a GaN layer formed between a sapphire substrate and GaN crystal can effectively suppress the threading dislocations and that a light emitting diode manufactured by using this technique can attain luminance more than ten times as large as that of a conventional light emitting diode.
The light emitting diode including the buffer layer of a GaN layer inserted between the sapphire substrate and the GaN crystal disclosed in Japanese Laid-Open Patent Publication No. 4-297023 will now be described with reference to FIG. 15.
As is shown in FIG. 15, the light emitting diode includes a buffer layer 101 of undoped GaN and a device structure 102 having a doublehetero junction structure successively stacked on a sapphire substrate 100. The device structure 102 includes an n-type GaN layer 103 working as a first cladding layer, an undoped IN.sub.0.2 Ga.sub.0.8 N layer 104 working as an active layer and a p-type GaN layer 105 working as a second cladding layer successively stacked. The device structure 102 is partially removed by dry etching so as to bare the inside of the n-type GaN layer 103. On the p-type GaN layer 105, a p-type electrode 106 is formed, and on the etched portion of the n-type GaN layer 103, an n-type electrode 107 is formed. The sapphire substrate 100 has a thickness of 150 .mu.m and the device structure 102 has a thickness of 50 .mu.m.
The present inventors manufactured a light emitting diode by a method described in Japanese Laid-Open Patent Publication No. 4-297023. Owing to the buffer layer 101 inserted between the sapphire substrate 100 and the device structure 102, the occurrence of the threading dislocations and cracks was suppressed in the device structure 102, but still there remained threading dislocations of approximately 1.times.10.sup.10 cm.sup.-2.
Thus, although the occurrence of the threading dislocations and cracks can be suppressed by the buffer layer 101 inserted between the sapphire substrate 100 and the device structure 102, the suppressing effect is still disadvantageously limited.